This invention relates to a high-frequency generator, comprising a multigrid electron tube having at least one grid pair, consisting of a control grid and a screen grid, a tube supply circuit and a load, both being connected to the main current path of the electron tube, and a feedback loop. The feedback is connected to the main current path of the electron tube on the one side and to either of the grids of the grid pair on the other, for generating a high-frequency signal in the main current path.
Such a high-frequency generator, which finds its application in industry, for example, for high-frequency heating, is described in the U.S. Pat. No. 4,761,618 dated Aug. 2, 1988.
The high-frequency generator described therein comprises a low-voltage control unit which is connected to a tetrode grid. By means of the low-voltage control unit the voltage level of a control voltage on the relevant grid can be adjusted thereby influencing the output power of the electron tube to be dissipated in the load.
The disadvantage of influencing in this fashion the output power of the electron tube by means of the low-voltage control circuit is the fact that during the operation of the generator conditions may arise in which the electron tube is damaged.